The employment of light that is highly transmissive by an organism and exhibits a peak in light intensity in a wavelength band ranging from a visible region to a near-infrared region permits noninvasive measurement of intracorporeal information. This is attributable to the Lambert-Beer's law defining that a logarithmic value representing a measured light signal is proportional to the product of an optical length by a concentration. A technology of evolving the law and measuring a relative change in the concentration of intracorporeal oxidized hemoglobin (Hb), a relative change in the concentration of intracorporeal deoxidized hemoglobin (Hb), and a relative change in the concentration (hereinafter, a concentration change) of total hemoglobin (Hb) (sum total of the oxidized Hb and deoxidized Hb) has been developed.
A technology of simultaneously and non-invasively measuring a Hb concentration change at multiple points in the human cerebral cortex on the basis of the above technology has been proposed (Non-patent Document 1: “Spatial and temporal analysis of human motor activity using noninvasive NIR topography” by A. Maki et al. (1995)(Medical Physics 22, pp.1997-2005)), and has prevailed in the fields of research and clinical practices. The document 1 has disclosed a method of measuring an Hb concentration change in the cerebral cortex so as to measure the cerebral functions of a human being. Specifically, along with the activation of the human sensor functions or kinetic functions, a blood volume in a cerebral region inside the cerebral cortex, which is responsible for the functions, locally increases. This causes the concentration of oxidized Hb or deoxidized Hb in the region to change, whereby the activities of the brain can be evaluated.
The technology is characteristic of being noninvasive and less-binding to a subject and permitting ready measurement of cerebral functions. Moreover, since the technology can evaluate the kinetics of blood circulation or the state of blood circulation, it is expected to be applied to an unprecedented medical field (clinical application). Examples of clinical applications that have reportedly proved useful include diagnosis of epilepsy (Non-patent Document 2: “Noninvasive cerebral blood volume measurement during seizures using multi-channel near-infrared spectroscopic topography” by E. Watanabe et al. (J. Epilepsy 11, pp.335-340) and identification of a language-significant hemisphere (Non-patent Document 3: “Noninvasive assessment of language dominance with near-infrared spectroscopic mapping” by E. Watanabe et al. (1998) (Neurosci. Lett. 256, pp.49-52).
In recent years, application to psychiatry has been encouraged. In patients suffering mental diseases such as depression and schizopherenia, Hb concentration changes observed through optical organism measurement reportedly exhibit a pattern different from the pattern exhibited by Hb concentration changes in sound people (Non-patent Document 4: “Multi-channel near-infrared spectroscopy in depression and schizopherenia: cognitive brain activation study” by T. Suto et al. (2004) (Biol. Psychiatry 55(5), pp.501-511). In the document 4, oxidized Hb is used as an index, and waveforms specific to subjects suffering from respective diseases are presented. However, the reproducibility in an individual is uncertain. It is not easy to accurately assess the presence or absence of a disease in each subject or the symptom thereof. Moreover, whether individual differences in each Hb concentration change stem from differences in commitment to a task (psychological factor) or differences in the kinetics of blood circulation (physiological factor) has not been clarified.